The present invention relates to plastic preforms or parisons and relates, in particular, to heat treatment of parisons to condition the parisons for further treatment or further forming such as blow molding.
Classically, a series of parisons are conveyed continuously which being rotated on a support mandrel through an oven containing a battery of heating elements. The heating elements impart heat to the body of the parison usually (but not necessarily) from top to bottom of a vertically disposed parison to condition plastic from which the parison is molded for subsequent treatment.
Invariably, the heating elements are fixed in position within the oven and are arranged relative to the path traversed by the parison and relative to the shape of the parison to create a desired temperature profile throughout the body of the parison.
That is, one heating element of a vertical array of elements is positioned a first distance from a first point or first region on the body of the parison while another heating element of the array is positioned a second distance from a second point on the body of the parison thereby creating a temperature profile and establishing a temperature gradient in the body of the parison.
A prior art device showing a battery of heating elements for conditioning the parison is disclosed and described in U.S. Pat. No. 4,690,633 to Schad et al. Reference to this patent shall be made a part of the file of this application by filing an Information Disclosure Statement accompanying the filing of this application.
In prior art devices, of which the '633 disclosure is typical, there are disadvantages arising from inability to (1) adjust the temperature profile readily and conveniently to satisfy the temperature requirements of a given parison structure, (2) to reset or readjust heating elements to satisfy requirements of an entirely different run of parisons or (3) repeat an earlier set-up.
In one prior art arrangement with fixed electric heating elements, the temperature profile is adjusted by varying or changing the power to the heating elements. While this procedure is operative, it does not possess sufficient flexibility to meet with requirements of tapered conical or other unusual shaped parisons.
While operating an electric element at maximum rating from a fixed position, heat generated is frequently insufficient to provide the desired temperature profile. Moveover, operating at maximum rating for sustained intervals reduces the effective life of the unit resulting in increased manufacturing costs.
In other prior art devices, the heating elements are fixed for a given run of parisons and while such elements are adjustable, to and fro, relative to the parison, it is necessary to dismantle the oven virtually to perform laborious and time consuming steps to set-up, initially, and make numerous adjustments to fine tune the heat requirements of a given run of parisons or to reset and readjust the heating elements for entirely different parison configurations.
As indicated above, frequently it is necessary to alter original settings several times in order to gain the optimum temperature profile for a given parison configuration. Obviously, the need to dismount heating elements involving adjustment of threaded member or other piece parts to gain the proper temperature profile, is laborious, time consuming and costly.